This invention relates generally to gas turbine engines and more particularly to film cooled combustor liners used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and burned for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. Combustors used in aircraft engines typically include inner and outer combustor liners to protect surrounding engine structure from the intense heat generated by the combustion process. The combustor liners are cooled to meet life expectancy requirements.
Liner cooling is commonly provided by diverting a portion of the compressed air (which is relatively cool) and causing it to flow over the outer surfaces of the liners. In addition, a thin layer of cooling air is provided along the combustion side of the liners by directing cooling air flow through an array of very small cooling holes formed in the liners. These cooling holes are axially slanted in a downstream direction and typically all have the same circumferential orientation. This technique, referred to as multi-hole film cooling, reduces the overall thermal load on the liners because the mass flow through the cooling holes dilutes the hot combustion gas next to the liner surfaces, and the flow through the holes provides convective cooling of the liner walls.
In addition to the film cooling holes, combustor liners are typically provided with dilution holes. The dilution holes, which are considerably larger than the cooling holes, introduce dilution air into the combustion zone. The dilution air quenches the flames so as to control the gas temperature to which the turbine hardware downstream of the combustor will be exposed. The quenching also reduces the level of NOx emissions in the engine exhaust.
However, each dilution hole represents an area on the liner that lacks film cooling holes. Furthermore, the wake produced by the influx of air through the large dilution holes will disrupt the cooling film behind them. This means that the regions of the liners immediately downstream of dilution holes can experience a loss of cooling film effectiveness. Thus, while film cooling of combustor liners is generally quite effective, the presence of dilution holes can result in hot spots being formed immediately downstream thereof. Over time, the hot spots can induce cracking in the liners, thereby reducing their service life.
Other common liner structural features, such as borescope holes and igniter ports, can disrupt the cooling film and cause hot spots in a similar manner. Cooling film effectiveness can also be weakened by causes other than such structural features. For instance, the flow of compressed air into the combustor is ordinarily swirled to enhance air-fuel mixing. These swirling combustor gases can disrupt cooling film in certain regions of the liners and create hot spots.
Accordingly, there is a need for a combustor liner in which cooling film effectiveness is increased in liner regions that are immediately downstream of disruptive structural features such as dilution holes, borescope holes and igniter ports or are otherwise subject to a loss of cooling film effectiveness.
The above-mentioned need is met by the present invention, which provides a combustor liner having a plurality of dilution holes and a plurality of cooling holes formed therein. The cooling holes include a first group of cooling holes angled in a first circumferential direction and a second group of cooling holes angled in a second circumferential direction, opposite to the first circumferential direction. The cooling holes in the second group of cooling holes are located between adjacent ones of the dilution holes. Groups of oppositely directed cooling holes could be used with other hot spot regions on the liner.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.